In the field of digital printing, a digital printer receives digital data from a computer and places colorant on a receiver to reproduce the image. A digital printer may use a variety of different technologies to transfer colorant to the page. Some common types of digital printers include inkjet, thermal dye transfer, thermal wax, electrophotographic, and silver halide printers.
It is a common goal in the field of digital printing to continually improve the quality of the output print, particularly when printing digital images of photographs. In recent years, advances in the technology related to digital printing have provided much opportunity for improving the quality of the output, particularly in the field of inkjet printing. An inkjet printer reproduces an image by ejecting small drops of ink from a printhead containing nozzles, where the ink drops land on a receiver medium (typically paper) to form ink dots. A typical inkjet printer reproduces a color image by using a set of color inks, usually cyan, magenta, yellow, and black. Often, the dots produced by the inkjet printer are visible to the human eye, and result in an undesirable noise or "grainy" appearance to the reproduced image. Modern inkjet printers typically reproduce images using smaller ink drops than their predecessors, thus reducing the visibility of the ink dots and therefore improving the image quality. Another technique employed by some modem inkjet printers to reduce the grainy appearance of reproduced images is to use multiple inks of the same color, but different densities, such as a light cyan and a dark cyan ink. The light ink dots are less visible to the human eye than the dark ink dots, and regions of the image reproduced with the light ink will appear less grainy than if a dark ink had been used to reproduce the same region.
One problem with using multiple inks of the same color to reduce the graininess of a printed image is that the total amount of ink that is placed on the page is increased. It is commonly known in inkjet printing that poor image quality can result when too much ink is printed on the page. For example, excessive ink can cause the receiver to physically warp or deform. Too much ink can also result in ink "bleeding" , where ink flows across the surface of the receiver from one area to another, resulting in undesired image artifacts and poor image quality. In addition to the potential poor image quality that results from using excessive ink, it is also more costly to operate the printer due to the greater ink usage. It is therefore advantageous to minimize the total amount of ink used to reproduce an image.
U.S. Pat. No. 4,604,654 to Sakurada et. al., describes a printer in which a light ink is used primarily in highlight regions of the image, and a dark ink is used primarily in the shadows, with the midtones being generally reproduced with a combination of the two inks. While this arrangement does provide for reduced image noise, it completely neglects the issue of the total ink amount that gets placed on the page, and could result in the undesired image artifacts described above. U.S. Pat. No. 5,142,374 to Tajika et. al., describes an inkjet et printer in which the light and dark inks are used with different dot sizes or resolutions. This art also provides for reduced graininess of the printed image, but does not consider the amount of ink that is being placed on the page, or the negative image quality artifacts that may occur. Related art describing similar printers are described in U.S. Pat. Nos. 4,635,078, 4,672,432, and 4,727,436, but the total amount of ink that is used is ignored in all of these references.